1. Field of the Invention
This invention relates to a semiconductor device comprising one or more semiconductor chips such as for example power MOSFETs and IGBTs built into a package.
2. Description of Related Art
Because semiconductor chips such as power MOSFETs and IGBTs are devices for controlling large currents, they produce large amounts of heat. Consequently, when these semiconductor chips are built into packages, it is arranged to achieve a sufficient cooling (heat radiation). For example, in the case of an IGBT module consisting of a plurality of IGBT chips built into a package, an insulating substrate made of a high thermal conductivity ceramic is used, and the plurality of IGBT chips are mounted on this insulating substrate, and main electrodes provided on the lower surfaces (lower principal surfaces) of the IGBT chips are connected by soft soldering to a copper thick film provided on the insulating substrate.
Main electrodes and control electrodes provided on the upper surfaces (upper principal surfaces) of the IGBT chips are connected to a copper thick film provided on the insulating substrate by wire bonding. The insulating substrate is soldered to a heat radiation plate made of copper. By this means, heat produced by the IGBT chips is transmitted through the insulating substrate to the heat radiation plate and radiated away. This kind of IGBT module is used in invertor main circuits of invertors in a class of several tens to several hundreds of Amperes.
In the case of an IGBT module of the related art construction described above, heat produced by the IGBT chips is radiated through the insulating substrate disposed on the lower surface side of the IGBT chips; that is, heat is radiated mainly from the lower surface of the IGBT chip. With this construction, because heat is only radiated from one surface of each of the IGBT chips, there is a limit to how much the heat-radiation performance can be raised, and reducing the size of the construction of the IGBT module as a whole has also been difficult.
When on the other hand the IGBT module is constructed so that heat is radiated from both the upper surface and the lower surface (the two principal surfaces) of the semiconductor chips, the heat-radiation performance can be increased greatly. One example of this construction is a thyristor package. This package has a structure wherein a thyristor chip is sandwiched by two electrode blocks serving both as electrodes and as heat sinks (radiator). With this construction, heat produced by the thyristor chip is transmitted from both the upper surface and the lower surface of the chip to the electrode blocks. In the case of a thyristor, to obtain electrical connection between the electrodes of the thyristor chip and the electrode blocks, the thyristor chip sandwiched by the electrode blocks is pressed with a considerably large force.
However, semiconductor chips like IGBT chips having MOS gate structures have the characteristic that they are vulnerable to stresses. Consequently, it is not possible to employ the method of pressing the semiconductor chips with electrode blocks. To overcome this, as a construction wherein semiconductor chips are sandwiched between two high thermal conductivity insulating substrates without being pressed, there is the construction disclosed in Japanese Patent Application Laid-Open No. S.59-31042. In this case of this Japanese Patent Application Laid-Open No. S.59-31042, because the lower side of the semiconductor chip is fixed to an electrode provided on an insulating substrate, heat produced by the semiconductor chip is radiated from this lower side of the chip smoothly.
However, on the upper side of the semiconductor chip, because the electrodes on this upper side and electrodes provided on the upper insulating substrate are connected by bonding pads and metal bumps, the area of the connection is small. Consequently, there has been the problem that the electrical resistance is large, which is disadvantageous to obtaining large currents, and that heat produced by the semiconductor chip is not readily transmitted to the insulating substrate, and thus the heat-radiation performance is poor.